Patent Grant
8312717
The present invention relates to externally heated engines. More particularly, the present invention relates to improvements in the efficiencies of externally heated engines operating at relatively low temperatures and pressures.
Externally heated engines and, in particular, Stirling cycle engines have always held great promise, because their theoretical thermal efficiency approaches that of the Carnot Cycle. This efficiency is established in turn by the difference between the hot and cold temperatures of the cycle. Recent designers of such engines have sought to maximize efficiency by increasing the temperature of the hot side of the engine. In addition, they have utilized fine molecule gasses, such as helium and hydrogen, at very high pressures, to further optimize the power output of the engine. Their combined efforts have resulted in commercial failure. The high temperatures have required the use of materials which can withstand these temperatures. The practical problems, and enormous expense, of using materials such as titanium and special alloys of stainless steels have combined to make the engines impractical to manufacture, and expensive to own and operate. High pressure gasses and extreme temperatures have made the engine so complex that it has been placed out of the reach of all but the most sophisticated users.
The present invention takes a completely opposite approach. Through the combined use of several innovations, the design of a high efficiency, low temperature, simple engine becomes possible.
To overcome the inefficiencies of engines of the past, the temperature differential between the air outside the cylinder, and the working fluid inside the cylinder must be very large to force the transfer of the necessary amount of heat in the very limited time available. This in turn forces the heat source itself to operate at an even higher temperature, and to be very tightly coupled to the heat exchanger. This tends to expose the external portions of the exchanger to even higher temperatures, which requires still more exotic materials.
An additional problem in the prior art engines concerns the temperature of the air sent to the regenerator. The extreme temperatures traditionally involved in the prior art make the use of common low temperature tubing, such as copper, impossible. This also applies to the materials used in the regenerator. Neither the outside of the regenerator or the material used in the regenerator matrix can be optimized for thermal performance, because the overriding concern is survivability at high temperature.
The problems of high temperatures completely dominate the design of a regenerator to be used in the prior art Stirling engines. This leads to significant thermodynamic losses, as well as greater expense, and reduced lifespan. The outside shell of the regenerator has to be made of high strength metals that will tolerate the high temperatures. This leads to high losses of heat to the environment, heat gained from the environment, and heat conducted from one end of the regenerator to the other. This heat conduction forces operation of the regenerator in a manner that is far from ideal.
The heat exchanger on the cold cylinder must efficiently remove heat from the working fluid, during the compression stroke. As with the hot side, prior art heat exchanger designs have used either the basic cylinder shape itself as the heat sink, or they have used simple finned surfaces or some variation of the shell and tube heat exchanger. In all such designs, the thermal resistance inherent in these approaches forces the heat sink to operate with a large difference in temperature (ΔT) between the interior and exterior of the cylinder.
In other words, the working fluid inside the cold cylinder is forced to be at a temperature considerably above the outside temperature at which the heat is finally dissipated. This greatly reduces the ΔT across the engine, which limits the maximum efficiency and power output of the engine.
Since the Stirling Cycle is a closed thermodynamic cycle, the working fluid must be sealed inside the engine. This leads to several major design problems.
First, the prior art engines are forced to operate at high temperatures and pressures. This places great demands on the seals. To survive the high temperatures and pressures, the only practical approach has been to use sealing rings on the piston, as in conventional internal combustion engines. The piston and ring assemblies suffer leakage, or blow-by. This fluid loss from the engine is a critical problem, as it must continually be replaced to avoid loss of power output, and it disturbs the cycle. This usually means that the crankcase itself must be sealed as well, leading to problems of lost work in the crankcase, as the pistons do unwanted work on the crankcase gas. It also means that the crankcase must be filled with the same working fluid as used in the engine itself.
The piston rings scraping up and down on the walls of the cylinder lead to further problems. The biggest of these is the friction created. In a typical engine this can consume some 20% of the engine's output, a very serious loss.
A further problem is that of lubrication. Liquid oils cannot be simply sprayed onto the cylinder walls, as this would leak into the working area of the engine and contaminate the working fluid. This would lead to problems involving unwanted contamination, corrosion, and loss of efficiency. But without adequate lubrication, the friction losses become even greater.
Another problem with engines of the past is that a large proportion of the working fluid does not move fully throughout the engine. An engine is needed which increases the amount of working fluid which participates in the process.
Additionally, an engine is needed which has a variable compression ratio to allow for maximizing the power output depending on the temperature of the heat source used to power the engine. An engine is also needed which has variable timing to optimize power output at various temperature, pressure and engine speed conditions.
The present invention solves all these problems found in the prior art designs.
Briefly described, the present invention includes an externally heated engine having a piston and a displacer. A piston reciprocates within a first cylinder. The piston has a first side (working side) and a second side opposite the first side. The first side of the piston and the first cylinder define a working chamber containing working fluid, which may consist of any usable gas. The second side of the piston and the first cylinder define an opposite chamber containing an opposing fluid.
A displacer reciprocates within a second cylinder. A heater heats the working fluid in the hot chamber of the cylinder of the displacer. Preferably, the chamber is heated by a heat source so that the working fluid has a temperature of no more than 500° Fahrenheit with a temperature difference between the heat source and the working fluid of less than 5° Fahrenheit. It is possible to generate useful energy with working fluid temperatures of less than 212° Fahrenheit. One source of heat is the cooling pools of spent nuclear fuel, which are below 212° Fahrenheit. Currently, this potential source of energy is released to the atmosphere and not used. The working fluid may be heated with a heat exchanger or heat injector. Heated fluid is delivered to the heat injector and flows through grooves around thermally conductive material, thus injecting heat directly into the engine. The heat is trapped inside the engine by the thermally insulating material. The working fluid flows in the longitudinal direction through the thermally conductive material. The thermally conductive material has passageways so that the working fluid may pass longitudinally through it. The longitudinal passages for the working fluid are narrow and run the entire-useable length of the heat injector.
Preferably, the heat injector has grooves for the heated fluid which include multiple, parallel grooves which form a spiral or helical pattern along the entire outside useable length of the heat injector. The spiral grooves could be in sets of 2, 3, 4 or more, running parallel to one another and into which the heated fluid is injected simultaneously. By keeping these grooves very narrow and deep, a very high value of length to depth and thus low temperature differential is achieved, while providing adequate useable cross-sectional area to permit a sufficient volume of heated fluid to flow and provide heat input. The temperature difference between the heating fluid and the metal of the heat exchanger will be only about 5 degrees Fahrenheit.
Preferably, the engine includes a diaphragm associated with the piston to separate the working chamber from the opposing chamber. The diaphragm provides many benefits as will be described in detail below. Because of the use of the diaphragm, it is beneficial to control the pressure of the opposing fluid. This prevents a large pressure differential across the diaphragm, which, if uncontrolled, could cause it to burst. A second reason is to vary the pressure on the opposing side in concert with the action of the engine's throttle control. That is, as working fluid pressure is raised and lowered, the same is done with the opposing fluid, to avoid doing unwanted work on the gas in the opposing chamber and to protect the diaphragm.
The working fluid pressure is controlled as a means of throttling the engine. As more working fluid is forced into the engine, by increasing its pressure with the control system, the engine will increase its power output, because the greater volume of working fluid will transfer more heat into and out of the engine cycle and thus do more work. Reducing the pressure will have the opposite effect. In this way, engine output can be continuously varied, to match the load conditions.
In displacer type engines, in order to force the greatest possible percentage of the working fluid in the engine to participate effectively in the thermodynamic process, as much fluid as possible must be swept alternatively all the way through the engine, from hot side to cold side and back again. This is obtained by making the volume of the displacer very large in comparison to the rest of the volume of the engine. This ensures that the vast majority of working fluid contributes effectively to the process.
The working fluid moves between the hot and cold chambers of the cylinders in a closed fluid path. A closed fluid path means that during normal operation, fluid reciprocates between the chambers, compared to a internal combustion engine, for example, which continually intakes combustion air and exhausts combustion byproducts to the atmosphere. The closed fluid path in the present invention does allow for the introduction of additional working fluid when necessary and for pressure.
A pressure differential is maintained between the working fluid and the opposing fluid in the first cylinder of between 5 to 35 psi. By maintaining pressurized opposing fluid, a higher working fluid pressure is possible while maintaining the integrity of the diaphragm. In addition, the opposing fluid aids in the compression stroke by reducing the work necessary to compress the working fluid. However, the pressure of the opposing fluid is not so high that it interferes with the power stroke. Preferably, the opposing fluid pressure is maintained between the minimum and maximum working fluid pressure. Ideally, the opposing fluid pressure is maintained at the mean of the minimum and maximum working fluid pressure. The externally heated engine may have the working fluid at a pressure of below 10 atmospheres. The externally heated engine may have the working fluid at a pressure of greater than 60 PSI.
A regenerator is provided within the closed fluid path. The regenerator is a temporary repository of heat during certain cycles of the engine. Because the temperatures are lower than in engines of the prior art, the present invention may employ a shell made out of polytetrafluoroethylene material. This material does not conduct heat. Thus there is no thermal short circuit around the mesh. In prior art regenerators operating at extremely high temperatures, only all-metallic internal components could be used. Since each layer of such metallic mesh touched both adjacent layers, a continuous, thermally conductive path was established from the hot side of the regenerator to the cold side. This resulted in a continuous loss of high temperature energy over to the cold side.
In the present invention, the regenerator operates at low enough temperatures to allow the introduction of non-metallic layers of mesh. Preferably, non-metallic mesh layers are used after every 10 or so metal layers. These non-conductive layers break up the conductive path, and thus prevent the unwanted loss of energy from the hot side to the cold side of the regenerator. In addition, since the non-metallic mesh layers can be made, for example, of woven fiberglass, they have enough thermal capacity to add slightly to the heat retention capacity of the regenerator, further adding regenerating action without adding unwanted, unswept volume.
Preferably, in addition to the metallic mesh layers and insulating mesh layers in the regenerator, a third type of layer is used. Specifically, a thicker, copper layer, which is solid with a pattern of larger openings can be used. The openings are arranged to break up and redistribute the air flow within the regenerator to ensure that the entire mesh content is fully utilized efficiently. The thicker copper also retains some additional heat, which adds further to the regenerating capacity. The regenerator does not need stainless steel wire in the mesh as with prior art regenerators, but may include copper wire, which is far more thermally conductive than steel. Silver may be used as an alternative to copper, for even higher performance. The copper mesh may be coated with diamond and may include a high melting point thermal insulating polymer such as polytetrafluoroethylene in the form of an outer cylinder and a center core. The regenerator may include a perforated disk constructed from a diamond copper composite. These choices allow the use of less mesh, with a consequent reduction in pumping losses. Alternatively, a regenerator with a series of tube segments separated by insulating material, such as, for example, fiber glass can be used. The tubes could be, for example, copper, silver, diamond coated metal, or combinations thereof.
The engine operates in the following manner. The heat applied to the hot side causes the working fluid, such as air, methane or another gas, to rise in pressure, and to expand. This forces the piston to move, thus doing useful work. The working fluid is then passed through the regenerator, on its way to the cold side. In the process it leaves behind much of its heat, which is temporarily stored in the regenerator mesh matrix. The fluid thus arrives in the cold side much reduced in temperature.
Once in the cold side, the fluid is compressed back to its original, smaller volume. This requires the removal of some heat, which is preferably rejected to a recuperator. This heat is thus recovered and reused.
Finally, the fluid passes back through the regenerator to the hot side. On the way it picks up the heat left behind in the regenerator mesh matrix. The fluid thus arrives in the hot cylinder at a much increased temperature and pressure. As further heat is added through the hot heat injector or exchanger, the fluid again enters an expansion process, thus beginning a new cycle of the engine. The piston and the displacer are arranged to reciprocate such that the volume of the working fluid is compressed and expanded alternately.
The externally heated engine may include a diaphragm attached to the piston to create a seal between the piston and the cylinder. The diaphragm may be a two layers of thin rubber separated by a woven mesh layer to increase the strength of the diaphragm. This diaphragm has virtually zero friction and zero break-away force. The diaphragm has a low melting temperature. Leakage is so slow as to be negligible. The unit is low cost, and will give up to a billion cycles in service.
The reason such a diaphragm can be employed in an externally heated engine is because of low temperature and pressure in the present invention. Without this, the high temperatures and pressures make the use of a diaphragm impractical. In prior art designs, a diaphragm would have to be made partly of thin, high temperature metals, with heat shielding. This would greatly increase friction and reduce service life, negating advantages of the diaphragm.
However, with the present invention, the diaphragm makes it possible to eliminate the main source of friction in the engine. That is, the piston rings are eliminated. A prior art Stirling engine will lose at least 20% of its output power to friction. The great majority of this friction is eliminated with the present invention. The diaphragm also eliminates the problem of leakage which is present with traditional piston ring seals. Because there is no leakage, the working fluid and opposing fluid do not mix, so that the working fluid does not become contaminated by the opposing fluid if those two fluids are not the same. The working fluid and opposing fluid need not be the same because of the perfect seal provided by the diaphragm. An opposing fluid such as dry nitrogen could be used, for example, to avoid oxidation and contamination of the volume enclosed in the bonnet. In addition, a light gas, such as helium, may be used as the working fluid, to obtain thermodynamic benefits, while still using a heavy gas such as air or nitrogen as the opposing fluid, thus avoiding the expense and difficulty of sealing the lighter gas on the opposing side, or providing quantities of it to make up for leakage.
Additionally, with the diaphragm, there is no need for lubrication in the cylinders, because the diaphragm is essentially frictionless. By eliminating lubricating oil, the working fluid does not become contaminated with lubricant.
In one embodiment of the engine, a piston is adapted for movement within a first cylinder. The piston has a first side and a second side opposite the first side. The first side and the first cylinder define a working chamber and the second side and the first cylinder define an opposite chamber containing an opposing fluid. A displacer is adapted for movement within a second cylinder. The displacer has a first side and a second side opposite the first side. The first side of the displacer and the second cylinder define a cold chamber and the second side of the displacer and the second cylinder define a hot chamber. There is a closed fluid path between the first and second cylinders which includes a working fluid. The working fluid is capable of moving between the working chamber, the cold chamber and the hot chamber. A regenerator is located within the closed fluid path. A heat source is provided for heating the working fluid. A source of cold could also be provided to cool the working fluid. A link is provided to cause reciprocation of the piston.
In some embodiments, a first disk is connected to a first shaft section and a second disk is connected to a second shaft section. A yoke can be connected to the first disk and the second disk and could be adapted for positional adjustment in the radial direction with respect to the first disk and the second disk. Preferably, the link is connected to the yoke such that positional adjustment of the yoke causes the position of the piston to change within the cylinder.
Preferably, the first cylinder has a first cylinder section and a second cylinder section, the first cylinder further includes a piston disk and a spacer disk. The piston is connected to the piston disk and the spacer disk is adapted to be attached between one of the first cylinder section and the piston disk and the second cylinder section and the piston disk. Preferably, there is a gasket adjacent the spacer disk. Additional spacers and gaskets could be used as needed. By placing spacers on one side or the other of the piston disk, the position of the diaphragm changes inside the cylinder, and thus the distance the diaphragm can travel before it hits top dead center changes. By changing the stroke of the piston, the amount of swept volume changes. Because the remainder of the volume of the engine is fixed, changing the position of the diaphragm changes the compression ratio of the engine, which is useful to accommodate various temperatures of the heat source for the engine.
In some embodiments, a displacer link is adapted to cause reciprocation of the displacer. The displacer link is connected to a displacer link disk. The displacer link disk is connected to a first shaft section and the piston link disk is connected to a second shaft section. The first shaft section and the second shaft section operably connected to each other. One of the shaft sections is operatively connected to a first disk and a second disk. The angular position of the first disk with respect to the second disk is adjustable. By adjusting the angular position of the first disk with respect to the second disk, the relative position of the displacer with respect to the piston changes. Depending on the temperature, pressure, revolutions per minute and other engine conditions, adjustment of the angular position can be used to optimize engine performance. In one embodiment, the first disk and the second disk include a plurality of bores and further include pins adapted to be inserted in the bores to fix the position of the first disk with respect to the second disk. In addition, threaded bores could be used for accepting bolts inserted through bores in at least one of the first disk and second disk.
In some embodiments, the displacer link is moved by a displacer cam assembly. The displacer cam assembly includes a first cam and a second cam. The first cam and the second cam each have a groove therein with a groove path. The displacer link is adapted to follow the groove path of the first cam and the second cam. The grove path in each of the first cam and the second cam varies in distance from the center of each cam. The displacer link can be connected to a pin and the pin can be inserted into the groove of each of the first cam and the second cam. In engines of the past, rotating crank disks impart a sinusoidal motion to the displacer. The cam assembly of the present invention can be configured to cause the displacer to dwell at the two ends of the stroke and to move rapidly from one end to the other. Preferably, the groove path has a first section which is at constant distance from the center of the cam. The groove path has a second section which is also at a constant distance from the center of the cam. The groove path makes a rapid transition between these two sections. When the pin is in each section of constant diameter, the displacer dwells at the ends of the stroke. When the pin is in the transition between these two sections, the displacer moves rapidly from one end point to the other. When the displacer dwells at the ends of its stroke, the thermodynamic cycle is squared off. By dwelling with the gas trapped in the hot end of the engine, during most of the power stroke, the gas is held as closely as possible to the ideal of an isothermal expansion with heat addition. Similarly, by causing the gas to dwell in the cold end during substantially the entire compression stroke, this part of the cycle approaches the ideal of an isothermal compression, with the heat of compression being removed.
In some embodiments, the displacer cold chamber includes a cooling element attached to the external surface of the cold chamber, which helps extract the heat of compression. In some embodiments, the displacer hot chamber includes a heating element attached to the external surface of the hot chamber, which helps replace the heat of expansion. Both the surface heating and cooling elements do not add any dead volume to the engine.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
a is a cross-sectional view of a portion of the stroke timing adjustment device of
b is a cross-sectional view of a portion of the stroke timing adjustment device of
c is a cross-sectional view of a portion of the stroke timing adjustment device of
d is a cross-sectional view of a portion of the stroke timing adjustment device of
Pushrod 312 is attached one end 314 to piston 112 and at the other end 316 to slider assembly 320. Slider assembly 320 is adapted for linear movement. Link 322 is pivotally connected to slider assembly 320 and allows for the conversion of linear motion to rotational motion. Bearings 326 guide the movement of pushrod 312.
As shown in
As the position of the link 322 is changed, the position of the piston 112 in the cylinder 114 must also be changed. The cylinder 114 is designed so that the location of the piston 112 can be changed with respect to the cylinder 114. As shown in
As seen in
a-13d illustrate the displacer 210 reciprocation cam assembly 610. The crankshaft section 422c has a cam 612 attached thereto. Similarly, the crankshaft section 422d has a cam 614 attached thereto. Each cam 612 and 614 has a groove 616 as best seen in
One of ordinary skill in the art will appreciate that there are many equally feasible power transmission methods and physical arrangements of the various elements described. The foregoing description is meant to provide a conceptual overview and should not be viewed as limiting the invention. While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
This application is a continuation in part of U.S. application Ser. No. 11/446,951, filed Jun. 5, 2006, which is a continuation of U.S. Pat. No. 7,076,941 the disclosures of which are hereby incorporated by reference herein.
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| Number | Date | Country | |
|---|---|---|---|
| 20090255249 A1 | Oct 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11198681 | Aug 2005 | US |
| Child | 11446951 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11446951 | Jun 2006 | US |
| Child | 12470539 | US |
